Current flat panel display devices can be classified largely into LCD (liquid crystal display) devices, PDP (plasma display panel) devices, and OLED (organic light emitting diode) display devices.
The PDP device has a simple structure and manufacturing process and thus provides an advantage in implementing large screens but, due to the low performance of the elements, may not be suitable for 3D display technology. Also, the active matrix LCD device has a high resolution and allows the implementation of large screens but may not be suitable for 3D displays. Compared with the above, the active matrix OLED display device applied with thin-film transistors (TFT) provides many advantages, such as realistic color reproduction rate, high contrast ratio, thin and light modules, low power consumption, high response speed, wide viewing angle, etc., and is thus gaining interest as a next-generation display device.
An OLED display device may use amorphous silicon thin-film transistors, polycrystalline silicon-based thin-film transistors, and oxide thin-film transistors, etc.
The amorphous silicon thin-film transistor has been considered with high priority in application to large-area active matrix OLED display devices, due to established manufacturing techniques and due to the property of the electron mobility being kept uniform over a large board.
However, the amorphous silicon thin-film transistor may be less desirable in terms of electrical stability due to the inherent properties of the amorphous silicon layer, and the continuous application of the gate bias may result in changes in the threshold voltage, possibly causing problems in driving the OLED pixel circuit within the active matrix.
That is, in order for the luminance of the OLED to be properly adjusted according to the applied data voltage (signal), the amorphous silicon thin-film transistor acting as the driving TFT for driving the OLED may have to operate in the saturation region, but a change in the threshold voltage of an amorphous silicon thin-film transistor may incur a change in the saturation region of the amorphous silicon thin-film transistor. Consequently, the current supplied to the OLED by way of the amorphous silicon thin-film transistor may be reduced, which in turn may lower the luminance of the OLED.
To resolve the problem above, some of the active matrix OLED display devices manufactured more recently use polycrystalline silicon thin-film transistors. The polycrystalline silicon thin-film transistor provides the advantage of enabling integration and allowing the implementation of SOP (system-on-panel) technology. However, the polycrystalline silicon thin-film transistor entails a high manufacturing cost, and the non-uniformity of the elements occurring during processing for a large-board display may generate large deviations in electron mobility.
Active matrix OLED driving circuits can be classified into voltage-driven circuits and current-driven circuits depending on the type of data inputted.
A current-driven circuit can simultaneously compensate for changes in the threshold voltage and for deviations in electron mobility that occur during normal processes, but due to the parasitic capacitance present in the wiring through which data is applied, the charging time can be quite long at low data current levels, creating restraints in the driving frequency.
A voltage-driven circuit allows easier charging and discharging compared to a current-driven circuit, so that it allows a higher operating speed and easier signal connection with the driving circuit of the display. As most driving IC's currently available employ the voltage-driven method, there may be no additional costs needed in manufacturing a driving IC.
However, in the case of the voltage-driven circuit, there have been various research efforts focused on compensating for the threshold voltage of a driving thin-film transistor, there has not been much research performed on compensating for the deviations in electron mobility in a driving thin-film transistor.